1. Field of the Invention
The invention relates to motor control and drive circuits. More particularly, it is directed to a motor control and drive circuit for controlling and driving motors, motor fans and the like while applied to motor controllers such as HDDs, FDDs, optical disk drives and other office automation equipment.
2. Description of the Related Art
FIG. 4 shows a three-phase motor controller in which a motor control and drive circuit is used. Such control and drive circuits have been proposed in U.S. application Ser. No. 07/998,058 and now U.S. Pat. No. 5,319,290.
In FIG. 4, reference numerals 31, 32 and 33 designate comparators; 34, a starter circuit; 40, a trapezoidal wave forming component pulse generation circuit; 41, 42 and 43, a sinusoidal wave approximated trapezoidal wave generation circuit; 44, 45 and 46, drivers; 80, a three-phase motor; and 80a, 80b and 80c, coils of the motor 80.
The comparators 31, 32 and 33 function as a sense circuit. These comparators compare outputs Ua, Va, Wa of the drivers 44, 45, and 46 with a neutral point voltage of the star-connected coils 80a, 80b and 80c to thereby detect a rotating condition of the motor 80. The comparators output detection signals Ub.sub.2, Vb.sub.2 and Wb.sub.2 whose phases are shifted from one another.
The starter circuit 34 has switching circuits SWu, SWv, and SWw. In a steady state, the switch circuits directly output the detection signals Ub.sub.2, Vb.sub.2, and Wb.sub.2 of the comparators 31, 32, 33 as detection signals Ub, Vb, Wb.
The use of a circuit corresponding to the sense circuit allows the rotating condition of the motor to be detected without using Hall elements or the like. It is for this reason that this type of motor is called a "Hall-less motor".
The trapezoidal wave forming component pulse generation circuit 40 generates three sets of trapezoidal wave forming component pulse groups Uc, Vc and Wc from the detection signals Ub, Vb and Wb by causing a gate circuit to perform logical operations. These pulse groups Uc, Vc and Wc are 120.degree. out of phase from one another.
The trapezoidal wave forming component pulse group Uc represents four trapezoidal wave forming component pulses A, B, C and D. The trapezoidal wave forming component pulses A, B, C and D are sequentially outputted in this order without being superposed one upon another. The same applies to the trapezoidal wave forming component pulse groups Vc and Wc.
The sinusoidal wave approximated trapezoidal wave generation circuit 41 generates a trapezoidal wave approximated to a sinusoidal waveform, or a sinusoidal wave approximated trapezoidal wave, in response to the trapezoidal wave forming component pulse group Uc in accordance with timings indicated by the respective component pulses. It is the sinusoidal wave approximated trapezoidal wave that is outputted as a drive signal U.
The driver 44 generates an output Ua having a current waveform following the waveform of the drive signal U by amplification, and outputs such output Ua to the coil 80a.
Similarly, a drive signal V is generated by the sinusoidal wave approximated trapezoidal wave generation circuit 42, and a drive signal W is generated by the sinusoidal wave approximated trapezoidal wave generation circuit 43. The phases of the signals V and W are shifted. The signals Va and Wa as amplified are outputted to the coils 80a and 80b.
Thus, the coils 80a, 80b and 80c are driven by the motor drive currents that moderately vary, thereby ensuring smooth rotation of the motor 80.
Such a Hall-less motor, not requiring that a detector such as a Hall element and an object to be detected being attached thereto, opens the way to downsizing and thinning the motor body, thus making itself requisite in the downsizing of office automation equipment.
The conventional Hall-less motor control and drive circuit feeds back the voltages generated at the coils of the motor in the steady state and generates the drive signal based on the feedback signal to maintain rotation.
However, at the time of starting the motor, voltage serving as the basis for a detection signal has not yet been generated at the coil. Therefore, a starter circuit that operates at the time of starting is required.
The starter circuit 34 generates oscillating signals Ub.sub.1, Vb.sub.1 and Wb.sub.1, which are 120.degree. out of phase from one another upon reception of a starting signal S. During starting, the switching circuits SWu, SWv and SWw are switched to forcibly replace the detection signals Ub, Vb and Wb by these oscillating signals.
In order to accelerate the rotation of the motor from the stop state to a predetermined rotational speed, the oscillating signals Ub.sub.1, Vb.sub.1 and Wb.sub.1 are generated so that the frequencies thereof are gradually increased. Once the predetermined frequencies have been reached, the switching circuits SWu, SWv and SWw are switched to return to the original condition.
When the rotational speed of the motor has been accelerated sufficiently by such operation of the starter circuit, the above-described steady state of the motor can be obtained thereafter by the operation of the feedback loop.
However, such starting control is achieved necessarily by an open loop control, not a control reflecting dynamic conditions of the controlled object such as feedback control. As a result, the setting of the frequencies of the oscillating signals Ub.sub.1, Vb.sub.1 and Wb.sub.1 and the timings at which the switching circuits SWu, SWv and SWw are operated must be adjusted to individual motor characteristics. If the adjustment is inadequate, the motor does not rotate at the time of starting and finds itself in an oscillating condition, or in an out-of-step condition in which local oscillation is repeated. As a result, the adjustment involves a number of steps, which imposes a problem in terms of impairing productivity.
Particularly, the out-of-step condition of the motor is attributable not only to variations in individual motors, but also to the stop condition and environmental conditions (load, temperature, inclination, etc.). The same motor may be subject to this trouble in a very small percentage even though the motor is free from such trouble normally. It is difficult to make proper adjustment of the motor, taking into account these accidental factors and secular changes.